Field of Invention
The present invention relates to frequency difference locking techniques, and more particularly to an optic-microwave frequency discriminator for laser frequency difference locking, and a method thereof.
Description of Related Arts
Precise frequency difference locking between two lasers are required in variety of applications such as distributed temperature/strain sensing based on Brilliouin scattering, photonic generation of high microwave frequencies. Recent years, DFB lasers with extreme narrow line width (a few KHz) are commercially available. However, the frequency difference between two independent free-running lasers are changing all the time and drifting in a range of a few ten MHz. The present invention is about a novel frequency difference locking technique.
A Brilliouin distributed sensor manufacture, OZ Optics, developed a proportional integral derivative (PID) controller for the frequency difference locking. A tunable fiber delay line is used for the frequency tuning. Fiber delay lines perform mechanical tuning, suffer from low-accuracy, low-speed and poor repeatability. Optical phase lock-loop is another approach for the control of frequency difference between two lasers. The technique requires expensive RF source and other RF components. Optical injection locking is a different technique for generating high quality microwave signals. It uses RF signal to modulate a master laser, then inject the high orders of the modulated spectra to lock two slave lasers. The technique, requiring RF source and a master laser, is complicated and very expensive. Microwave signal generation based on RF external modulation is based on use of higher orders of the modulation to generate higher microwave frequency. It requires only a single laser, but it still requires expensive RF source.
The present invention proposes a novel Optic-Microwave Frequency Discriminator (OMFD) to perform precise frequency difference locking between two lasers. The technique does not require any expensive RF sources. OMFD uses a microwave phase shifter for agile and accurate frequency tuning. It can adapt to different locking accuracy requirement and provides reliable, stable frequency locking and tuning.